Military aircraft are frequently equipped with head-mounted displays (HMDs) through which an operator of the aircraft, such as the pilot or navigator, may visualize real-world objects in the environment. The HMDs display virtual objects, such as tactical information, such that the operator may visualize the virtual objects superimposed over the view of the real-world objects. Newer HMDs are binocular in nature; that is, having two separate eye displays that are disposed over each eye of the operator.
Referring to FIG. 1, one prior art embodiment of an HMD 1 comprises a helmet 2, two separate left and right eye display screens 3a, 3b, two separate left and right image projection devices 4a, 4b, and a head tracking device 5. The display screens 3a, 3b, image projection devices 4a, 4b, and head tracking device 6 are typically mounted to the helmet 2. The display screens 3a, 3b take the form of semi-reflective mirrors through which the respective left and right eyes of the operator may see, and on which a two-dimensional image is projected via the respective image projection devices 4a, 4b, typically tactical information. The head tracking device 6 detects the position/orientation of the helmet 2, and thus the operator's head on which the helmet 2 is worn, and transmits this positional information to an HMD image generator 6 that is separate and remote from the helmet 2. The HMD image generator 6 generates a two-dimensional image that is transmitted to the image projection devices 4a, 4b for respective display of left and right images via the left and right display screens 3a, 3b. 
The HMD may either be used in the context of a real-world environment or a simulated virtual environment. For example, referring to FIG. 2, the HMD 1 may be used in a real-world environment, where the left and right eyes 7a, 7b of the operator can respectively see through the display screens 3a, 3b to the real world 8 outside of the canopy or windshield 9, such that the two-dimensional images 10a, 10b (in this case, “546 mph”) projected onto the display screens 3a, 3b, are seen by the operator as being superimposed over the real world environment 8. As another example, referring to FIG. 3, the HMD 1 may be used in a virtual environment inside of a simulator, where the left and right eyes 7a, 7b of the operator can respectively see through the display screens 3a, 3b to a screen 11 on which simulated images 12 generated by a computerized image generator (not shown) are displayed, such that the two-dimensional images 10a, 10b (in this case, “546 mph”) projected onto the display screens 3a, 3b are seen by the operator as being superimposed over the simulated images 12 on the screen 11. It is noted that the images 10a, 10b generated by the HMD 1 are virtual in nature in that they are not actually located on the respective display screens 3a, 3b, but rather appear distant (out in front of) the display screens 3a, 3b. 
Whether the HMD system 1 is located in a real-world environment or a simulated environment, it is important that the images being produced by the HMD are harmonized such that both eyes see the proper content for the type of display being configured (monocular, biocular, or binocular), and that the proper vergence condition is supported for the image distance. The two-dimensional images projected onto the display screens 3a, 3b are very similar and complementary to each other, or may even be identical depending on the type of HMD being supported. The images are in identical locations on the respective display screens 3a, 3b. The human brain will naturally process these identical images and see them as a single image. For this reason, it is important that the display screens 3a, 3b project their respective images into the same focal plane and more specifically into the same location in that plane. For example, as shown in FIGS. 4a and 4b, adjustment of the angles of the display screens 3a, 3b changes the angles of the optical axes 12a, 12b, thereby translating respective locations of the images 10a, 10b that are centered at the ends of the optical axes 12a, 12b. As shown in FIG. 4a, the images 10a, 10b are not converged together prior to adjustment of the display screens 3a, 3b, whereas the images 10a, 10b are converged to a fused image subsequent to adjustment of the display screens 3a, 3b. Even though the individual images 10a, 10b in the focal plane are not precisely coplanar in FIG. 4b, since each is orthogonal to the respective optical axis 12a or 12b, this deviation is well within the eye-brain's ability to fuse them together comfortably. The focal distances of the images 10a, 10b, which are virtual, have been designed or corrected by the HMD 1 to equal the viewing distance to the screen 11.
To this end, during fabrication of the HMD system 1 by a supplier, each of the display screens 3a, 3b will typically be mounted to the helmet 2 via a tilting mechanism (not shown), finely adjustable in two orthogonal axes via adjustment mechanisms, such as screws. Prior to shipping the HMD system 1 to a customer, the supplier may adjust the angle of the tilt platform via the screws to align the display screens 3a, 3b along a single focal plane.
Although this display mounting and adjustment process works well in theory, currently available tilt platforms employ multiple parts of considerable size and weight in support of the task of fine angular adjustment for the delicate display components. For example, current tilt platforms employ a set of two mounting plates separated by a ball or pedestal as a pivot point, which are held in close relationship with each using a captive spring arrangement of some kind for each axis of adjustment. Typically, these plates are fairly massy to contribute rigidity to the structure. The resulting size of the apparatus is too large to use in a small space, such as the available space in an HMD. Thus, for use in a HMD, the weight and size of the tilt platform may be excessive, thereby reducing the efficiency of the HMD. The use of multiple parts also increases the manufacturing cost of the tilt platform, and thus, the overall cost of the HMD.
There, thus, remains a need for a more compact, more lightweight, and less expensive dual-axis tilt platform for use in mounting a display in an HMD.